It is known that tumbling or vibrating parts to be surface finished (i.e. metals, plastics, etc.) in tubs containing numerous abrasive media "rocks" results in a smooth surface finish for the parts. The number of "rocks" used varies with the size of the tub and the number of parts to be finished, but may generally range from approximately 100 to 10,000 or more. The rotational or vibratory forces cause the abrasive media "rocks" and parts to interact and in effect deburr excess material from the parts and smooth rough spots on the parts. Water or other liquid compatible with the parts to be finished and the abrasive coated media rocks is usually added to the machine's tub to facilitate the process. The water or other liquid is continually or periodically drained during the machine's operation carrying with it waste from the media and the finished part.
Typically, media rocks used for this purpose have been made of hard and solid material such as natural stone, abrasive ceramic or metal. Although hard media is effective for some purposes, hard media is disadvantageous in that such media cannot conform to the shape of the parts to be finished, and hard media has difficulty imparting a fine, highly polished surface.
Some efforts have been made to employ flexible media, but such efforts have not be entirely successful. In U.S. Pat. No. 3,613,317, it is proposed to use abrasive impregnated resilient polymeric media whose hardness varies with the change in agitation temperature. This polymeric media is said to accommodate operations that require both high and low cut rate agitation by changing the water temperature added to the media. Thus, an extra step of obtaining high temperatures is needed in the agitation operation for the media to reach the soft state. Moreover, the temperature of a finishing operation is difficult to control in practice, and thus control of the desired finish is equally difficult. In addition, it has been found that this type of polymeric media can result in formation of a toxic sludge, which complicates disposal.
In U.S. Pat. No. 3,453,782, it is proposed to use media having a core of high specific weight material, such as a steel ball, surrounded by a resilient shell of a porous material, such as felt or foam rubber, the pores of which are filled with an abrasive material. It is said that the solid, high density core serves to provide the necessary weight for the media stones to effectively agitate with the parts and, in particular, to act as a solid bumper against which parts can bounce off upon contact, creating a "pumping" action which causes the particles imbedded in the shell's pores to rise to the surface. The core, however, acts as a detriment by limiting the size and shape of the media. Such media is also relatively heavy owing to the presence of the high density core, presenting storage and transportation difficulties. Furthermore, the fabrication of such media is relatively expensive, because each media "rock" consists of a solid core and two shell halves affixed together--essentially three parts for each "rock".